JPS6326559A - Foreign matter inspection device - Google Patents

Abstract

PURPOSE:To miniaturize a device and to stabilize the output light of a semiconductor laser by using the semiconductor laser which incorporates a sensor for output light monitor and performing feedback control with the output of the sensor. CONSTITUTION:The semiconductor laser 16 incorporates a photodiode 18 as the sensor for output light monitor. A driving circuit 22 supplies a current proportional to an input voltage to the laser 16, which emits light and the diode 18 receives a part of the output light of the laser 16 and outputs a current proportional to the quantity of the photodetected light to bring the laser 16 under feedback control. A sample holding circuit 24 holds the quantity of this feedback. Laser irradiation is performed opposite from right and left so as to detect anisotropic foreign matter stably. Further, a shutter which interrupts laser beam other than the output light of the laser 16 is provided. Then, the feedback control over the output light is performed in a state wherein no other laser beam is incident only right before foreign matter inspection, and the feedback quantity is held during the inspection to drive the laser 16 without using the output of the diode 18, thereby performing the stable foreign matter detection.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for stably maintaining the amount of illumination from a semiconductor laser at a set amount in a foreign matter inspection apparatus for semiconductor LSI wafers, photomasks, magnetic bubble wafers, etc.

[Conventional technology]

Inspection of foreign substances on patterned wafers during the intermediate process of LSI manufacturing is essential for improving product yield and reliability. The automation of this work is disclosed in Japanese Patent Application Laid-Open No. 55-149829.
In addition to the above publications, JP-A-54-101390, JP-A-55-94145, and JP-A-56-30630 are also available.
As shown in the above publication, this is realized by a detection method using polarized light. This principle is illustrated in Figures 10 to 1.
This will be explained using Figure 2.

As shown in FIG. 10, if only the illumination light 4 is irradiated onto the surface of the wafer 1 at an inclination angle φ, the pattern 2 and the foreign matter 3
Since scattered light 5 and scattered light 6 are generated at the same time, it is not possible to distinguish and detect only foreign matter 3 from pattern 2. Therefore, a device has been devised to detect only the foreign matter 3 by using polarized laser light as the illumination light 4.

As shown in FIG. 11(α), a pattern 2 existing on a wafer 1 is irradiated with five-polarized laser light 4. (Here, the case where the electric vector 10 of the laser beam 4 is parallel to the wafer surface is 5
It is called polarized laser illumination. ) In general, the surface irregularities of the pattern 2 are microscopically small enough compared to the wavelength of the illumination light and optically smooth, so the reflected light 5 also has an S-polarized component 1.
1 is maintained. Therefore, if the analyzer 13 that blocks S-polarized light is inserted into the optical path of the reflected light 5, the reflected light 5 is blocked and does not reach the photoelectric conversion element 7. On the other hand, as shown in FIG. 11(h), the scattered light 6 from the foreign object 3 includes a P-polarized light component 12 in addition to the S-polarized light component. This is because the surface of the foreign object 3 is rough, and as a result of depolarization, the P-polarized light component 12 is generated. Therefore, if the P-polarized light component 14 passing through the analyzer 13 is detected by the photoelectric conversion element 7, the foreign object 3 can be detected.

Here, as shown in FIG. 12, by using two laser light sources 15 from the left and right sides, it is possible to stably detect foreign objects that generate anisotropic scattered light.

As the laser light source 15, H, -N, and lasers have conventionally been used from the viewpoint of output light intensity, lifespan, polarization characteristics, and the like. However, since the oscillator body of Ha He laser is large,
This was inconvenient for downsizing the inspection device.

[Problem that the invention seeks to solve]

Since the above-mentioned conventional technology uses H, -N, and lasers as laser light sources, there is a problem in that the device cannot be miniaturized.

An object of the present invention is to use a semiconductor laser as a laser light source to make it possible to downsize the device and to stabilize the output light of the semiconductor laser.

[Means for solving problems]

The above object is achieved by using a semiconductor laser as a laser light source. In addition, the feedback control of the output light is performed only immediately before inspecting for foreign objects without the opposing semiconductor laser output light entering, and during the inspection, the amount of feedback is held and the output of the semiconductor laser is controlled without using the output of the monitoring photodiode. By driving the , stable foreign object detection becomes possible.

[Effect]

The time required to inspect the entire surface of the sample is several minutes at most. The ambient temperature change during this period is small, and if the feedback amount is held, the output light of the semiconductor laser will be stable during the inspection, so fluctuations in the output light of the semiconductor laser caused by the surface condition of the sample will be eliminated, and foreign particles will be detected. Fluctuations in detection performance are eliminated.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawing numbers.

Recently, semiconductor lasers have been improved in output power and have a longer lifespan, and have become fully usable as laser light sources for foreign object inspection equipment. When downsizing a foreign matter inspection device, it is effective to use a semiconductor laser as a laser light source.

The optical output of a semiconductor laser easily changes due to changes in ambient temperature. In order to use it as a laser light source for a foreign object inspection device, the optical output must be stable at all times. In order to keep the optical output constant even when the temperature changes, the optical output of the semiconductor laser 16 is branched by a half mirror 17, detected by a photodiode 18, and fed back to the driving current in the drive circuit as shown in Fig. 5. It is common to place it on the An example of the drive circuit is shown in FIG. This circuit controls the driving current of the semiconductor laser 16 so that the output current of the photodiode 18 (that is, the optical output of the semiconductor laser 16) is always constant. The setting of the optical output can be changed using a semi-constant resistor 19.

Most recent semiconductor lasers have a built-in photodiode for monitoring optical output. In this case, the external half mirror 17 and photodiode 18 shown in FIG. 5 are not necessary, and the function shown in FIG. 6 can be provided using a built-in monitor photodiode. Also, a dedicated I for semiconductor laser drive using feedback.
C is also commercially available.

The output light of an H, -N, laser is parallel light, but the output light of a semiconductor laser has a certain spread. Therefore, when used for illumination for foreign object inspection, it is necessary to use an objective lens or the like to condense the light, as shown in FIG.

As mentioned above, in order to stably detect anisotropic foreign matter, laser illumination is performed facing each other from the left and right sides. In this case, since the inclination angles φ of the laser beams 21'*21k are equal on the left and right sides, the opposing laser beams (for example, 21h) are reflected on the surface of the sample 1 and enter the inside of the semiconductor laser (for example, 16a). The amount of laser light that enters this interior is
It varies depending on the surface condition of the sample I (surface roughness, pattern density, etc.). If the semiconductor laser 16 has a built-in photodiode 18 for monitoring the amount of light, it monitors its own output light and at the same time monitors the output light of the other side that enters, so the set output light does not change. Feedback control becomes impossible. In other words, the output of the semiconductor laser 16 differs for each sample 1, and in the case of a patterned sample, the surface condition and pattern density differ depending on the location on the sample 1, so the output of the semiconductor laser 16 differs even within the same sample. It turns out that it changes.

Furthermore, as disclosed in Japanese Patent Publication No. 57-53923, a method is known in which a laser beam is irradiated onto a sample at a predetermined angle and the focal position is detected based on the position of the reflected light. Also in this case, as shown in FIG. 8, a semiconductor laser can be used as the laser light source. The output light of the semiconductor laser 16 is focused by a lens and irradiated onto the sample 1. The laser beam reflected by the sample 1 is focused by the condensing lens and detected by the sensor.

When the height direction position of the sample changes, the reflected light position changes, so
The sample position can be determined by detecting the position of the reflected light using a position sensor or the like. At this time, the reflected light from the surface of the sensor tool returns to the semiconductor laser 16 through the same optical path and is detected by the monitoring photodiode. It has been found that since the amount of the returned light varies depending on the surface condition of the sample 1, the output light of the semiconductor laser 16 also varies in this case.

Next, specific examples of the present invention will be described.゛Figure 1 shows a circuit for holding the amount of feedback. The drive circuit 22 supplies a current proportional to the input voltage to the semiconductor laser 16.
to make it emit light. The monitor photodiode 18 is
It receives part of the output light from the semiconductor laser 16 and outputs a current proportional to the amount of light received. The current-to-voltage conversion circuit converts the output current of the photodiode 18 into a voltage and supplies it to the sample and hold circuit. When the sample signal 5 is ON, the sample/hold controller outputs the input signal as it is, and when it is OFF, it holds the output signal section. The differential amplifier n outputs a voltage proportional to the difference between the output of the sample and hold circuit and the set value 26. 29 is a switch for turning on/off the semiconductor laser output light.

By using the above configuration for each semiconductor laser, when the sample signal 5 is ON, a feedback control loop is formed, and the output signal section of the sample and hold circuit is controlled to be equal to the set value 26 of the semiconductor laser 16. is driven. In other words, even if the ambient temperature changes, the output light of the semiconductor laser 16 can be controlled to be constant. When the sample signal 6 is OFF, the feedback amount immediately before turning OFF is held, and the semiconductor laser 16 is driven.

By turning on the sample signal '15 at least once within a certain period of time, the output light of the semiconductor laser 16 can be kept constant.

FIG. 2 shows the flow of foreign matter inspection, and normally a loop of wafer loading, foreign matter inspection, and wafer unloading is repeated. In other words, there is always a state in which the wafer is not set up in the inspection equipment just before foreign matter inspection, and at this time the output light from the opposing semiconductor laser does not enter the monitoring photodiode, making it possible to accurately monitor its own output light. . Therefore, during this period, the semiconductor laser output light can be stabilized at the set value by turning on the sample signal just before the wafer is loaded and keeping the sample signal off while the wafer is being set. becomes possible. As mentioned above, the time required to inspect for foreign matter is several minutes at most, and changes in ambient temperature during this time can be ignored, so that foreign matter inspection can be performed using a - constant semiconductor laser output light.

There are cases where the next foreign particle inspection is performed without unloading the wafer after the previous foreign particle inspection.

An example of this is when the same wafer is inspected multiple times as shown in FIG. In this case, the switch 29 shown in FIG. 1 is used. First, turn off the right semiconductor laser 16b.
F, turn on the semiconductor laser 16a on the left side, and turn on the sample signal on the left side. Next, the semiconductor laser 1 on the left
Turn off 6α and turn on semiconductor laser 16h on the right side.
and turn on the left sample signal. Next, when the semiconductor lasers on both sides are turned on, the feedback amount of each semiconductor laser is held, so the output light can be stabilized at the set value even if the wafer remains set.

In addition, as shown in Figure 9, if you insert a shutter that blocks the output light, the output light of the opposing semiconductor laser and the return light reflected by the sensor will not enter, so you can use the sample signal with the shutter inserted. The same function as above can be achieved by turning on the shutter and removing the shutter from the optical path during inspection.

When the sample-and-hold circuit is constructed from an analog circuit, a problem arises in which the hold value attenuates if the hold time is long. In the case of foreign matter inspection, since the hold time is several minutes, it is better to use a digital sample-and-hold circuit as shown in FIG. A current/voltage conversion circuit n outputs a voltage proportional to the output of the monitoring photodiode 18. , 30 is an analog-to-digital converter, which converts the output of the current/voltage conversion circuit n into a digital signal when the sample signal is turned on. Conversion value is memo 11
31 G, and the contents of the memory 31 are constantly converted into analog quantities by the digital-to-analog converter 32 (thus, a sample-and-hold circuit without attenuation can be constructed).

As mentioned above, when the output light of the opposing semiconductor laser is not incident immediately before starting a foreign object inspection, feedback control is performed using the output of the monitor photodiode to adjust the semiconductor laser to the set value. By controlling the output light so that it becomes the same as the output light and holding the feedback amount during the inspection, it is possible to obtain stable output light without being affected by the opposing semiconductor laser. Although the above description has been made regarding foreign matter inspection of wafers, the same applies to photomasks and magnetic bubble wafers.

〔Effect of the invention〕

As explained above, according to the present invention, the semiconductor laser output light can be stabilized during the inspection, so that even if the sample is different, a constant foreign object detection performance can be maintained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a feedback amount hold circuit according to an embodiment of the present invention, FIGS. 2 and 3 are diagrams each showing a flow of wafer foreign object inspection, and FIG. 4 is a diagram showing a sample hold circuit according to the present invention. 5 is a diagram showing a method for monitoring the output light of a semiconductor laser, FIG. 6 is a diagram showing a semiconductor laser drive circuit, FIG. FIG. 9 is a diagram showing an example of detecting the height of the output light, FIG. 9 is a diagram showing an example of blocking the output light with a shutter, and FIGS. 10 to 12 are diagrams for explaining the prior art. It is. 1... Wafer 16... Semiconductor laser 1
8...Photodiode n...Current/voltage conversion circuit Sae...Sample/hold circuit 22...Semiconductor laser drive circuit 35...Shutter representative Patent attorney Katsuo Ogawa, -Nishi 1 Tsutomi 2 Figure 3 times t4- Kaiki I5 Hai 2 Ra Kaimine 81 yen 1 Foot 9 figure J IO times

Claims (1)

[Claims] 1. Semiconductor laser driving means for performing feedback control on a semiconductor laser incorporating a sensor for monitoring output light using the output of the sensor; means for holding the amount of feedback; What is claimed is: 1. A foreign matter inspection device comprising: illumination means in which a plurality of optical means for irradiating output light onto a sample are installed facing each other. 2. The illumination means further includes a shutter means for blocking laser light other than the output light of the semiconductor laser, and before the inspection, only the output light of the semiconductor laser is monitored and fed back by operating the shutter means. 2. The foreign matter inspection apparatus according to claim 1, wherein the semiconductor laser is driven by controlling the shutter means to remove the shutter means and hold the feedback amount during the inspection. 3. The illumination means further includes means for individually turning on/off the plurality of semiconductor lasers, and holds the feedback amount after performing feedback control by turning on only one semiconductor laser except during inspection. The method according to claim 1, wherein said operation is performed for all other semiconductor lasers, and during the inspection, each semiconductor laser is driven using the held feedback amount of each semiconductor laser. Foreign matter inspection equipment.